When a pedestrian is hit by a motor vehicle, for example a car, one type of injury can be caused by a subsequent collision between the pedestrian's head and the hood of the vehicle. Many impact protection systems have been devised to reduce the effects of such collisions, for example hood-mounted airbags or energy-absorbing hood panels. Such deployable systems will be referred to as cushioning devices.
Cushioning devices require a sensor to be used to detect pedestrian impacts, and it is highly desirable for that sensor to discriminate between impact with a pedestrian and other types of impact. The decision of whether to deploy or not to deploy must be made in a very short space of time after detecting an initial impact at the front of the vehicle.
One pedestrian impact sensor system which has been proposed is described in International Patent Application No. WO 97/18108. This system uses a first sensor on the front bumper (fender) and a second sensor on the front edge of the hood of the vehicle. By measuring the time difference between triggering of the first sensor and triggering of the second sensor, and the magnitudes of the signals from those sensors, the system can distinguish between impacts with pedestrians and other sorts of impacts.
It is necessary for there to be an impact between the pedestrian and the vehicle hood before any deployment of a safety device can be triggered.